The present invention relates to a battery charging system that is suitable for use in a hybrid electric vehicle (HEV).
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing said method.
Hybrid electric vehicles have been put to practical use in recent years and include a generator for generating electric power by being driven with an engine, a high voltage battery (also called vehicle drive battery or traction battery), a vehicle drive motor, etc. Hybrid electric vehicles are roughly classified into three types: (1) a parallel type in which driving wheels are rotated by using the driving force of the engine and the driving force of the vehicle drive motor in combination; (2) a series type in which driving wheels are rotated by the vehicle drive motor driven with electric power generated by driving the engine; and (3) a combination type of them.
In addition to the high voltage battery with a relatively high voltage (e.g. 600 V), HEV:s are further equipped with a low voltage battery (electric equipment battery) for storing a DC power that has a relatively low voltage (e.g. 24 V). This low voltage battery is used to operate vehicle-mounted electric equipment, which includes lamp equipment (such as head lamps, stop lamps, etc.), air-conditioning equipment (such as an air-conditioning compressor, capacitors, etc.), audio equipment (such as a stereo set, etc.), control equipment (such as various controllers, brake vacuum pumps, etc.), and so forth.
FIG. 1 discloses an example of a known electric circuit of the electric equipment power source unit of a conventional series type hybrid electric vehicle. As shown in the figure, a vehicle drive motor 110 is connected to driving wheels 111 so that power can be transferred for propulsion. The vehicle drive motor 110 is further connected to a high voltage battery 103 through an inverter 109. Power to the driving wheels is controlled by a motor controller 108.
A generator 102 is connected to an engine 101 so that it can generate electric power by being driven with the engine 101, the operation thereof being controlled by a generator controller 107. The generator 102 is further connected to a high voltage battery 103 through an inverter 109 so that the generated electric power is supplied to the high voltage battery 103.
A low voltage battery 106 is connected to the high voltage battery 103 through a DC/DC converter (voltage converter) 104, and to vehicle-mounted electric equipment 105.
Therefore, if the electric power of the high voltage battery 103 is supplied to the vehicle drive motor 110 through the inverter 109, the vehicle drive motor 110 is rotated and the driving wheels 111 connected with the vehicle drive motor 110 are rotated, whereby the vehicle can travel. If the electric power stored in the high voltage battery 103 is reduced, the engine 101 is driven and the generator 102 is operated by the generator controller 107. The electric power generated by the generator 102 is accumulated and stored in the high voltage battery 103.
The electric power stored in the high voltage battery 103 is converted to a low voltage by the DC/DC converter 104 and stored in the low voltage battery 106. The vehicle-mounted electric equipment 105 is operated by the supply of electric power from the low voltage battery 106.
In FIG. 1 the conventional auxiliary power source circuit is provided with an alternator 112 that generates electric power by being driven with the engine 101. The alternator 112 is connected with the low voltage battery 106 in parallel with the DC/DC converter 104. With this arrangement, electric power is supplied from two systems (which consists of or comprises the alternator 112 and DC/DC converter 104) to the low voltage battery 106. This technique is disclosed for example in US2006/0232238 and Japanese Laid-Open Patent Publication No. Hei 10-174201.
The output voltages of the DC/DC converter 104 and alternator 112 are basically constant, but the voltages of their actual charging circuit sections vary with the magnitude of equipment load or state of the low voltage battery 106.
As a charging path from the power supply source to the low voltage battery 106, there are two paths: (1) a first path where the low voltage battery 106 is charged by the DC/DC converter 104 to which electric power is supplied from the generator 102 and (2) a second path where it is charged by the alternator 112 driven with the engine 101.
A problem with said prior art technology is that when too much energy (power) is transmitted in and/or out of the high voltage battery in a HEV the endurance of the battery is reduced. The endurance of the high voltage battery is also reduced by deep discharging.
It is desirable to reduce the effect of the above problem and to provide an improved battery charging system for a hybrid electric vehicle.
The battery charging system for a HEV comprises (includes, but is not necessarily limited to): a high voltage battery for supplying electric power to a vehicle drive motor; a generator for charging said high voltage battery; a DC/DC converter for converting a high voltage to a low voltage; a low voltage battery that is charged with said low voltage from said DC/DC converter; and an alternator, which is driven by an engine, connected with said low voltage battery in parallel with said voltage converter. The invention is characterized in that a control unit is arranged to register a parameter, where said parameter is at least one of a:                state of charge of said high voltage battery,        energy inflow/outflow to/from said high voltage battery to said vehicle drive motor or said generator;and where said control unit is arranged to control said low voltage in dependence of said registered parameter.        
The advantage with the system according to the invention is that endurance of the high voltage battery can be extended without decrease in vehicle driving performance. The solution is cost effective and easy to implement.
According to one embodiment of the battery charging system according to the invention, said control unit is arranged to control the low voltage of the DC/DC converter to be higher than a voltage from said alternator when state of charge of said high voltage battery is high.
According to another embodiment of the battery charging system according to the invention, said control unit is arranged to control the low voltage of the DC/DC converter to be higher than a voltage from said alternator when an inflow of energy to said high voltage battery is registered.
According to one embodiment of the battery charging system according to the invention, said control unit is arranged to control the low voltage of the DC/DC converter to be lower than a voltage from said alternator when state of charge of said high voltage battery is low.
According to another embodiment of the battery charging system according to the invention, said control unit is arranged to control the low voltage of the DC/DC converter to be lower than a voltage from said alternator when an outflow of energy from said high voltage battery is registered.